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Deposition Overview for Microsystems Primary Knowledge Participant Guide Deposition Overview for Microsystems Primary Knowledge Participant Guide Description and Estimated Time to Complete Deposition is the fabrication process in which thin films of materials are deposited on a wafer. During the fabrication of a microsystem, several layers of different materials are deposited. Each layer and each material serves a distinct function. This unit provides an overview of the deposition processes and the various types of deposition used for microsystems fabrication. This learning module introduces you to the common processes used to deposit thin films in the fabrication of micro-size devices. Activities provide further exploration into these processes as well as the properties of the thin films deposited. Estimated Time to Complete Allow at least 20 minutes to complete this unit. Southwest Center for Microsystems Education (SCME) Deposition Overview PK Fab_PrDepo_PK00_PG_March2017.docx Page 1 of 23 Introduction Microsystems (or MEMS) are fabricated using many of the same processes found in the manufacture of integrated circuits. Such processes include photolithography, wet and dry etch, oxidation, diffusion, planarization, and deposition. This unit is an overview of the deposition process. The deposition process is critical for microsystems fabrication. It provides the ability to deposit thin film layers as thick as 100 micrometers and as thin as a few nanometers.1 Such films are used for • mechanical components (i.e., cantilevers and diaphragms), • electrical components (i.e., insulators and conductors), and • sensor coatings (i.e., gas sensors and biomolecular sensors) The figure below shows a thin film of silicon nitride being used as the diaphragm for a MEMS pressure sensor. MEMS Pressure Sensor close-up (Electrical transducers (strain gauges) in yellow, Silicon nitride diaphragm in gray) [Image courtesy of the MTTC at the University of New Mexico] Because thin films for microsystems have different thicknesses, purposes, and make-up (metals, insulators, semiconductors), different deposition processes are used. The deposition processes used for microsystems include the following: • Spin-on film • Thermal Oxidation (oxide growth) • Chemical vapor deposition (CVD) • Physical vapor deposition (PVD) • Electroplating This unit provides a brief overview of deposition and each deposition method. More in-depth coverage can be found in additional instructional units. Southwest Center for Microsystems Education (SCME) Deposition Overview PK Fab_PrDepo_PK00_PG_March2017.docx Page 2 of 23 Objectives • Briefly describe two (2) deposition processes. • Create a chart that illustrates the type of thin films deposited which each deposition process. Key Terms (These terms are defined in the glossary at the end of this unit) Chemical vapor deposition (CVD) Deposition Electroplating Evaporation Oxidation Physical vapor deposition (PVD) Sputtering What is Deposition? Deposited Thin Films for MEMS Structure [Image courtesy of Khalil Najafi, University of Michigan] Deposition is any process that deposits a thin film of material onto an object. That object could be a fork, a door handle or, in the case of microsystems, a substrate. It is one of the primary processes in the construction of microsystems. Prior to the photolithography and etch processes, a solid, thin film of material is deposited on the wafer. For microsystems, this thin film is a few nanometers to about 100 micrometers thick.1 Southwest Center for Microsystems Education (SCME) Deposition Overview PK Fab_PrDepo_PK00_PG_March2017.docx Page 3 of 23 What is the Purpose of a Deposited Layer? Layering for MEMS Switch [Khalil Najafi, University of Michigan] The actual thickness and composition of the film is dependent on its application within the device. There are several different functions for thin films within microsystems fabrication. Here are some typical layers. • Structural layer (used to form a microstructure such as a cantilever (above), gear, mirror, or enclosure) • Sacrificial layer (deposited between structural layers, then removed, leaving a microstructure like the cantilever in the above graphic) • Conductive layer (usually a metal layer that allows current flow) • Insulating layer (separates conductive components) • Protective layer (used to protect a portion of another layer or the entire device) • Etch stop layer (used to stop the etch of another layer when a cavity depth or a membrane thickness is reached) • Etch mask layer (A patterned layer that defines the pattern to be etched into another layer) Southwest Center for Microsystems Education (SCME) Deposition Overview PK Fab_PrDepo_PK00_PG_March2017.docx Page 4 of 23 Type of film vs. Application Different films are used for various applications: Type of Thin Film Applications Silicon Dioxide (oxide) • Sacrificial Layer • Masking Material Polysilicon (poly) • Structural material • Piezoresistive material Silicon Nitride (nitride) • Electrical isolation between structures and substrate • Protective layer for silicon substrate • Environmental isolation between conductive layer and atmosphere • Masking material • Structural material Phosphosilicate Glass (PSG) • Structural anchor material to the substrate • Sacrificial Layer Various metals (Aluminum, gold, • Conductive electrodes platinum) • Reflective material Spin-on Glass (SOG) • Final layer for planarized top surface Zinc Oxide (ZnO) • Active piezoelectric film • Sacrificial layer Photoresist • Masking material • Sacrificial material Table 1: Type of Thin Film vs. Application Southwest Center for Microsystems Education (SCME) Deposition Overview PK Fab_PrDepo_PK00_PG_March2017.docx Page 5 of 23 MEMS Deposition Processes Polysilicon structural layer (the cantilever structure), Silicon nitride (isolation), Gold adhesive layer, probe coating (chemically reactive layer to sense specific particles) The goal of deposition is to achieve a high quality, thin, solid film on the substrate surface. Since microsystems fabrication requires different layers for different purposes, deposition could occur many times during the fabrication of a MEMS. The graphic shows four layers used for a microcantilever sensor: cantilever structure, silicon nitride, gold, and probe coating. Each layer requires a specific deposition process to deposit the specific film of a desired thickness. The most commonly used deposition processes for microsystems include the following: • Spin-on film • Thermal Oxidation (oxide growth) • Chemical vapor deposition (CVD) • Physical vapor deposition (PVD): Evaporation and Sputtering • Electrodeposition (electroplating/electroforming) Following are brief discussions of each of these processes. Spin-On Deposition Spin-on deposition is the process of literally spinning a liquid onto the wafer surface. The thickness of the film is dependent upon the liquid’s viscosity and spin speed. Once the liquid is spun onto the wafer, the solvents within the liquid are thermally evaporated through a curing process. The result is a thin, solid film. Spin-on deposition is used primarily for photoresist and spin-on glass (SOG). A more detailed discussion of the spin-on process can be found in the SCME Photolithography Overview. Spin-on Photoresist Layer Southwest Center for Microsystems Education (SCME) Deposition Overview PK Fab_PrDepo_PK00_PG_March2017.docx Page 6 of 23 Thermal Oxidation Thermal oxidation is the process used to grow a uniform, high quality layer of silicon dioxide (SiO2) on the surface of a silicon substrate. Thermal oxidation is different from other types of deposition in that the silicon dioxide layer is literally "grown" into the silicon substrate. Other types of deposition "deposit" the layer on the substrate surface with little to no reaction with the surface molecules. Silicon Dioxide Two silicon dioxide layers used as sacrificial layers for MEMS structure This graphic depicts the use of silicon dioxide for two different layers. The first layer (or bottom green layer) uses thermal oxidation to grow the silicon dioxide on the silicon substrate (see the discussion on Thermal Oxidation Process). The second oxide layer (the top green layer) is deposited using chemical vapor deposition (CVD). Silane gas and oxygen are provided and combined to form the silicon dioxide (oxide) layer. (More on CVD later in this unit.) Both of these oxide layers are considered sacrificial because they are subsequently removed to create the free, moving components of this structure. Silicon dioxide is a high-quality electrical insulator. It can be used for a variety of purposes: • A barrier material or hard mask • Electrical isolation • A device component • An interlayer dielectric in multilevel structures • A sacrificial layer or scaffold for microsystems devices. Silicon wafer with a layer of silicon dioxide Southwest Center for Microsystems Education (SCME) Deposition Overview PK Fab_PrDepo_PK00_PG_March2017.docx Page 7 of 23 Thermal Oxidation Process When a silicon substrate is exposed to oxygen, the silicon surface oxidizes to form a layer of silicon dioxide (SiO2). The amount of oxygen available, the source of the oxygen (gas or vapor), temperature, and time determine the final thickness of the oxide layer. This process is analogous to rust growing on iron. Rust is iron oxide and is formed by a chemical reaction between iron and oxygen. The amount of rust is dependent upon the temperature and humidity of the surroundings. For example, rust grows faster and thicker in hot, humid environments than in cool, dry environments. Loading silicon wafers into
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